1. Field of the Invention
This invention relates generally to microdocument viewing apparatus and more particularly it relates to a compact microimage reader with means employed along its optical path to substantially eliminate any keystone distortion of the image.
2. Description of the Prior Art
Along with the growing need to compact large volumes of information, e.g., on microfilm, there has been a commercial need for compact and lightweight viewing apparatus to make these compacted volumes of information readily available to the user. This growing commercial need has resulted in several compact viewing apparatus which have attempted to produce highly illuminated images with a minimum amount of image distortions. These prior art apparatus have generally been plagued with one or more of the following problems: (1) "hot spots," i.e., formation of areas of excessive light intensity at the center of the viewing surface; (2) overheating of the apparatus housing; (3) the general bulkiness of the reader; and (4) the overall high cost of the apparatus.
In order to achieve the maximum compactness in the usual prior art viewing apparatus which employs a narrow-angle projection lens, i.e., a lens which projects a beam over a full angle of about 30.degree. or less, several mirrors or other reflective surfaces are required. For example, one prior art viewing apparatus making use of a narrow-angle projection lens required a telescopically arranged two-part housing structure to contain the reader. The reader consisted of a main housing portion which contained a light source, a microform receiving station and a narrow-angle optical projection system. A second housing portion contains two mirrors, swingably mounted with respect to each other and the housing, and a screen member to receive light reflected from the two mirrors. While such prior art apparatus can be made to appear compact when not in use, it requires substantial space during operation.
In the viewing apparatus art it is desirable that the image be projected onto a light-distributive screen in such a manner that the image will appear to be rectangular and evenly illuminated to the viewer. However, in attempts to maximize on compactness of the apparatus, some viewing apparatus have made use of a light-distributive screen which has been positioned at an acute angle to the projection lens rather than perpendicular to the lens along its optical path; and in such apparatus, the projected image will have the undesirable feature of being keystoned, i.e., image projected onto the edge of the screen closest to the projection lens appears smaller to the viewer than does the image projected onto the farthermost edge of the screen.
Where the light-distributive screens of the prior art apparatus have been of the type having non-matte surfaces (e.g., highly directional flat aluminum foil screens), areas of excessive light intensity, i.e., "hotspots," occur on the viewing surface, if light is projected onto the screen in a manner producing divergingly deflected rays.
Several prior art apparatus have attempted to solve the problems of keystoning, "hot spots," etc. incurred in designing compact apparatus by making use of several mirror surfaces in the projected light path (e.g., to eliminate keystoning) and by spherically curving the screen to converge the light towards the center of the screen (e.g., to eliminate "hot spots"). While such prior art apparatus is not necessarily plagued with the problems of keystone distortion or "hot spots," the apparatus outer housing is limited in its minimum size.